These studies are directed at establishing the relation between the functional characteristis of small-diameter primary afferent fibers, their neural connections, and the mechanisms with which they are associated. The particular focus is on the functional organization of the sensory systems related to pain. One set of experiments concentrates on primary sensory units with fine myelinated or with unmyelinated afferent fibers, relating their functional characteristics to the spinal regions where their central processes terminate. Another group of experiments concerns the connections made by neurons of the spinal marginal zone and substantia gelatinosa, including investigation of how cellular morphology relates to the functional characteristics of cells in these areas. A third group of experiments is directed at the effects that sympathetic activity produces upon the responsiveness of cutaneous sense organs. All projects involve studying functional activity through electrophysiological recordings of single neural elements and determining their structural characteristics by light and electron microscopic identification of the neuron or neuronal process recorded from.